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20 15 a a? E10 a) 0 5 E Z Year Figure 6. Number of female salmon per redd for the years 1970-1986 above the Leaburg Diversion Dam. The average of 8.5 females per redd indicates that redd superimposition is occurring be- cause of spawning gravel limitations. (No data were collected in 1984.) channels (Figure 10). With less fre- quent mobilization of the bed, salmon now spawn in the main chan- nel in addition to the tributaries (James and Deverall 1987). The more stable channels provide more shel- ter for fry at high flows. Greater amounts of spawning gravels and higher levels of fry survival during the rearing period may be the rea- sons why the salmon population in the Waitaki is now larger than that in any other New Zealand river. However, this benefit to the salmon may come at the expense of a wading bird so endangered that less than 100 individuals remain. Once widespread, the black stilt (Himantopus novaezealandiae) is now restricted to the Waitaki River drainage (Pierce 1984). The stilts nest exclusively within the river channel on the large exposed bars isolated from the shore by the vari- ous braids of water. Vegetation that has proliferated on and stabilized the gravel bars also provides cover for introduced predators, such as ferrets (Mustela furo) and feral cats (Fells catus), which have exacted a tremendous toll on the adult stilts, their eggs, and nestlings (Pierce 1986). This increased predation pressure may have contributed to the stilts abandoning the lower Waitaki since 1960 as a stilt nesting area. The few remaining black stilts breed only in the Waitaki catchment above the dam. This example illus- trates how geomorphic changes af- fect different species of riverine plants and animals differently, with the potential for manifold alterations of ecosystem processes and species interactions. A geomorphic perspective These examples only hint at the va- riety and complexity of the geo- morphically driven ecological re- sponses of rivers to dams. On the McKenzie, Oconee, and Waitaki Rivers we discussed only the most conspicuous changes, those directly related to high-profile species. In addition, if the discharge of sedi- ment and water had been altered in a different way (as would likely have happened if the dams had been built for a different purpose), the re- sponses might have been different. And while the three rivers we exam- ined are fairly dissimilar, they by no means encompass the diversity of river form and process. Depending on the type of river and the type of dam, major geomorphic responses may include incision or aggrada- tion, change in channel pattern (e.g., braided or wandering rivers becom- ing single-thread rivers or vice versa), the streambed becoming coarser or finer, channel widening or narrow- ing, increased or decreased lateral migration of channels, loss of ripar- ian vegetation, riparian encroach- ment in active channels, and bank collapse. Geomorphological adjustments can lead to such ecological conse- quences as changes in the flux of nutrients and energy and the alter- ation of the habitat for riparian veg- etation, periphyton, invertebrates, and fish. These habitat changes may occur at scales ranging from sub- strates to riffles and pools to flood- plain features. While there are many ecological problems directly associ- ated with dams (e.g., blocked mi- grations and changes in tempera- ture, reduced or fluctuating flows; see Stanford and Ward 1979), it is our thesis that identifying and, if possible, minimizing or mitigating the physical geomorphic changes may often be crucial to protecting the biological integrity of a river. The gross effects of dams on downstream channel morphology, such as channel incision or aggrada- tion, have long been noted (Leopold et al. 1964, Simons 1979, Williams and Wolman 1984). While most of the early concern focused on the civil engineering implications of these changes, such as the scour of bridge abutments (e.g., Hammad 1972), the potential biological sig- nificance of these changes was rec- ognized some time ago (Petts 1980). A common solution to perceived deleterious changes in channel mor- phology (often viewed from a fish- eries-habitat perspective) has been to add structures to streams or to provide habitat through reengi- neering of the channel (see Petts et al. 1989 and Swales 1989 for sum- maries of these techniques). A more satisfactory alternative is to attempt to maintain the natural morphol- ogy of rivers below dams by man- aging water releases and sediment in ways that preserve, as much as possible, the pre-dam geomorphic processes. This approach originated with gravel flushing flows prescribed for regulated rivers in which the peak discharges have been reduced to the point that the bed rarely, if ever, moves. Flushing flows are high flows that are used to mobilize the bed and purportedly remove accumu- lated fine sediments in fish (prima- rily salmonid) spawning gravels (Reiser et al. 1989). Flushing flows have evolved into the channel main- tenance-flow concept and, more re- cently still, into floodplain and val- ley maintenance-flow concepts (Hill et al. 1991, Rosgen et al. 1986). Those who are developing mainte- nance-flow concepts are attempting to find a generalized procedure for deriving flow schedules for regu- lated rivers that will serve to main- tain geomorphic processes and pre- vent commonly noted deleterious morphological changes below dams, such as encroachment of riparian vegetation into the stream channel or the filling of pools with sedi- ments. Derivation of a flow regime is essential, but we believe that it is unlikely that a general method can be found that is applicable to all or even most streams, because the nec- essary flow regime depends criti- cally on the geomorphic conditions 188 BioScience Vol. 45 No. 3 1970 1975 1980 1985